After back to back saves in his first two appearances with the Mets in 1973 Hennigan struggled the rest of the way. In 30 appearances Hennigan posted an 0-4 record with 3 saves and a 6.23 ERA. After his appearance on July 7, in which he allowed 3 runs over 2 innings, Hennigan was sent down to the minors. It would be his last appearance in the majors. He finished the season pitching for the Tidewater Tides. After once again pitching for the Tides in 1974, Hennigan retired.
While with the Mets Hennigan wore number 34.
Phil Hennigan also played for the Cleveland Indians from 1969-1972.